The present invention relates to an improved grey scale technique and driver circuit for an active matrix thin film electroluminescent device.
Traditional thin-film electroluminescent (TFEL) displays are normally constructed of a laminar stack comprising a set of transparent front electrodes, typically made of indium tin oxide, formed on a transparent substrate (glass), and a transparent electroluminescent phosphor layer sandwiched between front and rear dielectric layers situated behind the front electrodes. Situated behind the rear dielectric layer are rear electrodes oriented perpendicular to the front electrodes. To illuminate an entire display, each row electrode is sequentially selected and scanned, and column electrodes are simultaneously energized with voltage pulses to illuminate selected pixels in a row. All rows are scanned in turn until the entire display has been illuminated, thereby writing a frame of video data. The period for doing this is sometimes referred to as a frame time.
For a monochrome display, a grey scale is a desirable feature in order to provide better screen clarity and definition. Current techniques to achieve a grey scale for passive thin-film electroluminescent displays can be broadly categorized as the modulation of the amount of charge flow through the phosphor layer. Present modulation techniques may be divided into four subcategories, namely, amplitude modulation, pulse width modulation, frame averaging, and spatial dithering. These techniques have been used with traditional electroluminescent displays to achieve the grey scale.
Amplitude modulation is the modulation of the magnitude of the voltage pulses imposed across the electroluminescent layer. Different voltage pulse magnitudes within the operating range of the electroluminescent layer (typically 120 volts to 220 volts), cause different pixel brightness. Within certain limits a higher voltage pulse causes more light to be emitted than a lower voltage pulse. Amplitude modulation provides reasonably accurate results but accurate control of the column driver output voltage is difficult to maintain. The control circuitry required to provide reasonably accurate amplitude modulation of the applied voltage waveforms entails a substantial amount of circuitry, which in turn requires substantial silicon area to fabricate at increased expense.
Pulse width modulation is the control of the time duration of a voltage pulse imposed across the electroluminescent layer during each frame to control the amount of light emitted from the pixel. The total luminescence from a pixel increases with the increased duration of the voltage pulse. However, the capacitive nature of thin-film electroluminescent displays results in shadowing in the display when using pulse-width modulated waveforms. Also, thin-film electroluminescent displays are less sensitive to pulse width modulation than to amplitude modulation, thus achieving a grey scale display with pulse width modulated signals can lead to undesirable display artifacts.
Frame averaging may also be used to generate multiple grey levels in a thin-film electroluminescent display. Frame averaging is the illumination of pixels during different subframe time periods. The average illumination during each of the subframes results in the desired total luminescence output from each pixel within a frame. An example of such a technique is disclosed in U.S. patent application Ser. No. 08/383,902, assigned to the same assignee, and incorporated herein by reference. The electroluminescent layer inherently has a relatively fast refresh response time requiring the refresh rate of the display to be extremely high, when employing a large number of grey levels, to avoid flicker.
Another grey level technique is to spatially dither the data to create the illusion of a greater number of grey levels. Spatially dithering is usually undesirable because of the decreased resolution and other artifacts due to the coarseness of the image.
What is desired, therefore, is a grey scale technique that provides high luminance output, accurate grey levels, and minimizes display artifacts.